Dysregulated generation of effector cells and their cytokines often results in the development of inflammatory- related diseases such as diabetes, inflammatory bowel disease, multiple sclerosis, the spondyloarthropathies and systemic lupus erythematosus (SLE). Inappropriate responses to cytokines following antigen receptor activation during acute infections can skew lymphocyte development toward memory versus effector cells resulting in an inability to resolve infections. To further our understanding of underlying disease mechanisms mediated by cytokines and to generate new and improved therapies, it will be necessary to continue to dissect how cytokine gene expression and function are regulated during an inflammatory response. Recently we have shown that miRNAs have a significant impact on immune system functions. Specifically, miR-155 exerts its control on the immune response this control, in part, by regulating cytokine production. However, the molecular mechanisms by which miR-155 controls cytokine production have not been fully appreciated. Using our miR- 155 mutant mouse model, we believe our preliminary data offers the first evidence that miRNAs control the expression of specific inflammatory cytokines by controlling chromatin remodeling. We show: 1) miR-155 regulates the production of specific effector cytokines, including IL-17 and IL-22, but not IL-17F or IL-21 both of which also define the TH-17 lineage and function; 2) neither il-17 nor il-22 3' untranslated regions (UTR) contain predicted miR-155 target sequences, and miR-155 controls IL-17 and IL-22 production through a pathway distinct from the STAT3 and RORt/ROR transcriptional pathways; 3) mechanistically, we have demonstrated that the expression and phosphorylation status of the histone modifier HP-1 are compromised in miR-155-/- TH-17 cells, and by correcting the level of HP-1 in the mutant cells we could partially rescue the defective TH-17 cytokine expression program. More importantly, these observations were confirmed in human TH-17 cells. Hence, the following aims form the core of our proposal: 1) Determine how does HP-1 regulate TH-17 cytokine gene expression 2) Determine the impact HP-1 has on global gene expression during CD4+ T cell activation/differentiation. 3) Study the regulatory effects of HP-1 on immune functions and gene expression in vivo. These studies will provide the first and comprehensive understanding of the novel role of miRNAs such as miR-155 in the function of the epigenetic modifier HP-1 in immune cells, and the impact this control has on inflammatory cytokine gene expression as well as global gene expression. They will reveal the previously unknown regulatory mechanism of IL-17 and IL-22 gene expression that is distinct from the accepted paradigm. They will define the immunological impact of one miRNA, from epigenetic regulation of global lymphocyte gene expression to broader effects on immune pathologies, thus aiding in future drug design efforts targeting miRNAs.